The 3D in Super Mario 3D Land is a gimmick. But it's because it's just a gimmick that it actually works. In every other 3DS game, I always turn on 3D initially, then turn it off again minutes later. It's distracting when you move the screen a quarter inch and have everything go out of focus, and the effect never seems to add anything worth the extra eyestrain. But in Mario 3D Land, I kept coming back to 3D so often that I eventually left it on the whole time.
The reason that the 3D is a gimmick is because the camera rigidly forces you to view the game from specific angles. Giving you full camera control would work just as well, but if you're trying to sell a game on its 3D effects, Mario 3D Land shows the way to do it. The game is set in a 3D world, but you traverse it in the same side-scrolling way as New Super Mario Bros. and all the old Mario adventures -- just with a lot more leeway to move left and right. All that makes some of the jumps a little unnecessarily tricky, but you earn lives so frequently, dying is more of an inconvenience than an impediment to progress.
But because the game forces you to look at things from one angle, the 3D effects work to great effect. The game plays tricks with the perspective, and sometimes the only way to know whether something's above or below you is to turn up the 3D slider. Sure, you still have to keep your screen locked in one position at a specific distance from your eyes, but it feels like you can get a better feel for the way the world is laid out when you see it in 3D.
All this rambling about 3D doesn't mean you have to use it, though. If it's not your thing, you can tweak the camera slightly by toying with the directional pad (though that feels a bit clunky in the heat of platforming). And even the optical illusion puzzles, where you have to figure out how to make your way to a coin hidden in plain sight by navigating blocks that only make sense when you look at them in 3D, almost always includes a button that lets you see the puzzle from a different angle.
Beyond all that, Mario 3D Land is enjoyable because it's broken up into genuinely bite-sized chunks. The stages and progression through worlds are more like an assortment of challenges, and each one lasts just a few minutes -- perfect for a handheld. Screw up enough and the game even throws you extra items, an invincibility suit, or a chance to skip the level entirely. Sure, taking those items might feel like cheating, but they're just another choice that you can ignore completely. Power-ups and extra lives are plentiful, and, like every other recent Mario title, there's plenty of game left to explore even after the credits roll (and that's where things pick up in difficulty for the more seasoned Mario veterans).
Nintendo knows how to capture the nostalgia of their franchise, but still temper it with ideas that feel innovative. For every familiar bit of music or a level background that reminds you of Mario games past, you have new abilities to use and deviously designed platforming sections that feel completely unique. And for once, I actually preferred playing a game in 3D. Sure, it's still slightly tiring on my eyes and it significantly cuts down the length of time I can play at a stretch, but the short stages always make you feel like you're making progress, no matter how little time you invest. The tired, princess-saving formula never seems to change, but somehow Nintendo makes every visit to the Mushroom Kingdom feel like the first time.